Overview
Transmissive optical components used in high-average power (HAP) lasers experience significant heat load due to absorption of optical energy and other processes. This heat must be removed (often in real time) for the transmissive optical component to operate correctly and efficiently. For example, the process of frequency conversion in a nonlinear laser material generates heat within the nonlinear material medium due to absorption. This heat must be removed if the frequency converter is to operate efficiently at a significant power level. Also, the process of storing energy in a solid state laser amplifier material also generates heat within the laser medium that must be removed, especially if the amplifier is to operate at a significant input power. Other transmissive optical components subjected to heat load and requiring cooling include crystals used in Pockels cells and glass used in Faraday rotators.
Traditional Methods of Heat Removal
A traditional method of heat removal from solid state crystalline materials employed in laser systems is to remove the heat from the sides of the materials, in a direction transverse to the direction of laser energy propagation. The removal of heat in a transverse direction causes thermal gradients in this direction. This creates several problems. In general, temperature gradients generate thermal-optical stress and index variations, which in turn cause thermal aberrations that distort the laser beam. More specifically, in most frequency conversion materials, the temperature variation in a direction transverse to the direction of propagation of the laser beam must be maintained to within a very small tolerance range. The presence of a thermal gradient in this direction severely limits the aperture size and the power loading allowed in a laser system design. Transverse cooling is described in a paper entitled “The Potential of High-Average-Power Solid State Lasers,” by J. L. Emmett et al., Document No. UCRL-53571, dated Sep. 25, 1984, available from the National Technical Information Service, and hereby incorporated by reference into the present application.
Conventional beam shaping techniques have been used to cool crystals whereby the laser beam is optically flattened in one transverse direction. This allows the crystal to be cooled along a greater length, and reduces the path from the center of the beam to the edge of the crystal where it is cooled. However, this method is not practical in all applications and requires a relatively high degree of complexity in the associated optics.
In some crystalline materials, and in particular beta-barium borate (BBO), the direction of greatest thermal conductivity in the material is also aligned closely with the direction of optical propagation. In order to efficiently remove heat from materials with this property, the heat must therefore be removed from the optical faces. One method of face cooling is a convective process, normally achieved using a flowing gas. In this method a gas is forced at high velocity across the faces of the crystal. The chief disadvantage of this method is that it requires a complex, active cooling system, and is therefore less suitable for applications requiring low cost, weight and volume, and a high degree of reliability. Also the engineering to implement this method is complex because the gas flow across the optical surfaces must be very uniform to avoid optical distortion.
U.S. Pat. No. 5,363,391, entitled “Conductive Face-Cooled Laser Crystal”, and issued to Steven C. Matthews et al on Nov. 8, 1994 and hereby incorporated by reference, discloses and claims techniques for passively removing heat from an optical element in a laser system through its optically transmissive faces (FIG. 1). Heat is removed by way of optically transmissive heat conducting media disposed adjacent the optically transmissive surfaces of the optical element. Heat is transferred out of the optical element in a direction parallel to the direction of propagation of optical radiation, thus minimizing problems associated with thermal gradients. Devices employing optical elements such as nonlinear frequency conversion crystals and laser crystals may utilize this heat management approach to achieve better performance. Heat is transferred to the heat conducting media by direct contact or through narrow gas-filled gaps disposed between the optical element and the heat conducting media.
U.S. Pat. No. 6,330,256, entitled “Method and apparatus for non-dispersive face-cooling of multi-crystal nonlinear optical devices”, and issued to Robert W. Byren et, al on Dec. 11, 2001 and incorporated by reference herein, teaches how to use the face-cooling method taught in U.S. Pat. No. 5,363,391 with multiple nonlinear crystal formats used primarily for second harmonic generation without the need for air-path rephasing between the crystals (FIG. 2). One or more birefringent crystals are cut and oriented such that there is no dispersion between the fundamental and second harmonic wavelengths within each crystal. The birefringent crystals are then disposed in a heat-conducting housing, sandwiched between two or more nonlinear crystals and used as the face-cooling medium. The multiple crystal assembly may be further sandwiched between optically transmissive windows which need not be birefringent or non-dispersive, these windows being used to protect the outermost nonlinear crystals and/or provide additional face cooling. This causes the heat generated in the nonlinear crystals by absorption at the fundamental and second harmonic wavelengths to flow longitudinally (direction of beam propagation) into the face-cooling medium, thereby minimizing any transverse thermal gradient in the nonlinear crystals and the attendant dephasing loss. The crystals can be dry stacked with a very small gas-filled gap as taught in U.S. Pat. No. 5,363,391, immersed in a liquid or gel of suitable refractive index, bonded with suitable optical cement, optically contacted, or diffusion-bonded together to form a composite crystal.
The above-described systems and methods rely on heat transfer from the transmissive optical component to a heat sink by means of conduction due to a mechanical contact, optical contact, bonded joint, or a narrow gas-filled gap. It is well known, however, that transmissive optical components exposed to thermal load tend to warp significantly. Unless an external force is provided, the effective contact area in mechanically and optically contacted joints is, therefore, significantly reduced, which typically leads to increased temperatures and warpage. Bonded joints typically use organic adhesive which has a low thermal conductivity and, therefore, impedes effective heat transfer. In addition, bonded joints cause increased stresses in the transmissive optical component since its transverse thermal expansion is now constrained by attachment to a heat sink. Finally, heat conduction through narrow gas-filled gaps is rather limited even when gasses with high thermal conductivity are used. Because of the above limitations, there is a need for an improved method for cooling transmissive optical components in HAP lasers.